You should get it to avoid problems. I have been on Caribbean cruises where I have seen a parent with children barred from boarding the ship at the point of departure without proof of consent. Fortunately, they were just boarding at different times and the mother and children had to wait in the terminal a couple of hours for the father to meet them rather than waiting more comfortably on ship. I also once saw in an international airport a woman traveling with her children and her children were removed from her custody (separated from her) because they were a different last name and she had no proof she was their mother or that she had the consent of the father. Part of that was that the mother was being uncooperative and using abusive language on the airport staff. I never saw the end result of that as I had to move on, but everyone around knew what was going on as loud as the mother was yelling.
Be safe, get the consent.
FYI--from the Department of State's "Tips when Traveling abroad" web-site:
http://travel.state.gov/travel/tips/tips_1232.html
Some countries require that a traveler’s U.S. passport be valid at least six months or longer beyond the dates of the trip. In addition, with the number of international child custody cases on the rise, several countries have instituted passport requirements to help prevent child abductions. (Mexican law, for example, requires a child traveling alone, or with only one parent, or in someone else's custody, to carry written, notarized consent from the absent parent or parents if the child is not in possession of a U.S. passport.) Contact the embassy of the foreign destination for more information. A listing of foreign embassies and consulates in the U.S. is available on the Department of State’s website at
http://www.state.gov/s/cpr/rls/dpl/32122.htm. Foreign embassy and consulate contact information can also be found on the Country Specific Information for each country.
And from information about visiting Ireland. This isn't an official site, but it is specific to traveling in Ireland:
http://www.citizensinformation.ie/en/travel_and_recreation/travel_to_ireland/visas_for_tourists_visiting_ireland.html
If a child (under 18 years of age) is travelling alone, both parents/guardians must have given consent. If the child is travelling with one parent, the consent of the other parent is required. Where only one parent has total custody and access rights, you must provide official evidence of this.